Abstract. Sulphuric acid concentrations were measured and calculated based on pseudo
steady state model with corresponding measurements of CO, NOx, O3,
SO2, methane and non-methane hydrocarbon (NMHC) concentrations as well
as solar spectral irradiance and particle number concentrations with size
distributions. The measurements were performed as a part of the EU project
QUEST (Quantification of Aerosol Nucleation in the European Boundary layer)
during an intensive field campaign, which was conducted in Hyytiälä,
Finland in March–April 2003. In this paper, the closure between measured and
calculated H2SO4 concentrations is investigated. Besides that,
also the contribution of sulphuric acid to nucleation mode particle growth
rates is studied. Hydroxyl and hydroperoxy radical concentrations were
determined using a pseudo steady state box model including photo stationary
states. The maximum midday OH concentrations ranged between 4.1×105 to
1.8×106 molecules cm-3 and the corresponding values for HO2
were 1.0×107 to 1.5×108 molecules cm-3.
The dominant source
term for hydroxyl radicals is the reaction of NO with HO2 (56%) and
the reaction of CO with OH covers around 41% of the sinks. The sulphuric
acid source term is the reaction SO2 with OH and the sink term is
condensation of sulphuric acid. The closure between measured and calculated
sulphuric acid concentrations is achieved with a high agreement to the
measured values. In sensitivity studies, we used different values for the
non-methane hydrocarbons, the peroxy radicals and nitrogen dioxide. The best
fits between calculated and measured values were found by decreasing the
NO2 concentration when it exceeded values of 1.5 ppb and doubling the
non-methane hydrocarbon concentrations. The ratio, standard deviation and
correlation coefficient between measured and calculated sulphuric acid
concentrations are 0.99, 0.412 and 0.645, respectively. The maximum midday
sulphuric acid concentrations varied between 3×105 to
1.9×107 molecules cm-3 for the measurements and
3×105 to 1.4×107 molecules cm-3
for the calculations, respectively. An average
participation of sulphuric acid to the nucleation mode particle growth rates
is 8.8%. Classifying the days into two groups – ''polluted'' days with
air masses originated over Central Europe or UK, and ''cleaner'' days with
air masses originated over the Northern Atlantic or the Polar regions –
reflects an equal sulphuric acid contribution to the aerosol growth in both
air mass classes.